This application claims priority under 35 U.S.C. .sctn..sctn.119 and/or 365 to "THE CHOPPING POWER SUPPLY CONTROLLER", Application No. H9-142745 filed in JAPAN on May 30, 1997, the entire content of which is herein incorporated by reference. This invention relates to a chopping power controller for supplying electric power to an electric load. More particularly, this invention relates to a switching circuit for a chopping power controller for supplying electric power to a switched reluctance motor.
Referring now to FIGS. 14(a), 14(b), 14(c), 15(a), 15(b), 15(c), 16(a) and 16(b), a switching circuit is disclosed for a power supplying circuit which supplies an electric power to a switched reluctance motor. A switched reluctance motor (hereinafter SR motor) generally comprises a rotor having outwardly projected magnetic poles and a stator having inwardly projected magnetic poles. The rotor includes a laminated core with steel plates. The rotor includes coils wound around the magnetic poles. The rotor of the SR motor rotates when poles of the rotor are attracted by poles of the stator. Accordingly, in order to rotate the rotor in a desired direction, coils have to receive electric current in certain order depending upon a rotational position of the rotor. For an example, such conventional SR motor is disclosed in Japanese Laid Open Patent Publication HO1-298940.
In the conventional SR motor, magnetic attraction is rapidly changed due to the current being switched from one coil to another depending on the position of the poles of the rotor. The rapid changes of the magnetic attraction will cause relatively large mechanical vibration, which generates undesirable noises.
The Japanese Publication discloses a scheme to generate a rotational position signal with gradual rising and falling edges. The publication also discloses a scheme to supply electric current to the stator coils with gradual rising and falling edges based upon said rotational position signal. By such gradual changes of the electric current, the vibration and the noise may be reduced. However, in such a conventional scheme, the vibration and noise reduction may not be significant when the rotor rotates slowly because the rising and falling edges of the supplied current change rapidly due to the rotational position signal. On the contrary, in the conventional scheme, output torque may be deteriorated when the rotor rotates fast because the coils receive the electric current in a short time and rising and falling edges of the supplied current change gradually due to the rotational position signal. Efficiency and output torque of the SR motor may not be good enough unless the switching timing of the supplying current is regulated based upon the desired rotational speed and output torque.
Japanese Laid Open Publication Nos. H07-274569, H07-298669 and H08-1 72793 disclose pulse width modulation circuits for smooth transition of the electric current supplied to the motor and switching mode control for increase of output torque. For example, an H-shaped switching circuit supplies electric power to a coil 1a which is one of three coils of a three-phase motor. The switching circuit includes the first switching element 18a, the second switching element 18b, the first diode D1 and the second diode D2. The first switching circuit 18a is interconnected between one end of an electric coil 1a and the first power supplying line 18e. The second switching circuit 18b is interconnected between the other end of the electric coil Ia and the second power supplying line 18f. The first diode D1 is interconnected between one end of the electric coil 1aand the second power supplying line 18f so as to allow one-way electric current from the second power supplying line 18f to the electric coil 1a. The second diode D2 is interconnected between the other end of the electric coil i a and the first power supplying line 18e so as to allow one-way electric current from the electric coil 1a to the first power supplying line 18e. A current sensor detects the amount of electric current flowing through the electric coil 1a. The switching elements 18a and 18b are turned on when the detected current is less than a target value (Vr1). The switching elements 18a and 18b are turned off when the detected current exceeds a target value (Vr2). In other words, chopped electric power is supplied to the electric coil 1a based on the comparison among the detected current and the target values (Vr1, Vr2).
As shown in FIG. 14(a), an electric current flows through the electric coil 1a when the switching elements 18a and 18b are turned on. On the contrary, as shown in FIG. 14(b), a regenerative electric current flows through the electric coil 1a when the switching elements 18a and 18b are turned off. As shown in FIG. 14(c), a greatly waved current flows through the electric coil 1awhen the switching elements 18a and 18b are repeatedly turned on and off together. In this application, this switching mode is called "hard chopping". Under the hard chopping mode, the regenerative current is supplied to the first power supply line 18e to be quickly weakened when both of the switching elements 18a and 18b are turned off. In this configuration, the current varies greatly in response to the operation of the switching element 18a and 18b. Thus the attractive force applied to the rotor may be varied greatly due to the greatly waved electric current.
As shown in FIG. 15(c), less waved current flows the electric coil 1a when FIG. 15(a) and FIG.15 (b) are alternatively repeated. In FIG. 15(a), both the first and the second switching elements 18a and 18b are turned on. FIG. 14(a) is identical as the FIG. 15(a). In FIG. 15(b), the first switching element 18a is turned off and the second switching element 18b keeps turning on. FIG. 15(b) shows a state when a current flowing through the coil 1a is less than the second target value (Vr2) and exceeds the first target value (Vr1). In this application, such alternation of FIGS. 15(a) and 15(b) is called "soft chopping". Under the soft chopping mode, the regenerative current is gradually decreased while the first switching element 18a is turned off and the second switching element 18b is turned on. Therefore, driving force of the SR motor and radial attracting force between the rotor and the stator are also gradually weakened. Accordingly, less vibration and noise may be generated under the soft chopping mode.
Some conventional power controllers select one of the hard and soft chopping modes by referring the supplied current or rotational condition of the SR motor in order to achieve low vibration and high torque. For example, Japanese laid open patent publication No. H07-274569, H07-298669 and H08-1722793 disclose such conventional power controllers.
However, high frequency noise could affect a detected signal generated by the current sensor for the electric coil 1a. Such high frequency noise could be significant when an inexpensive and simple sensor is used. FIGS. 16(a) and 16(b) show typical high frequency noises. FIG. 16(a) shows such noise under the hard chopping mode. FIG. 16(b) shows such noise under the soft chopping mode. Under the hard chopping mode, relatively large noise is generated because the supplied current is greatly changed by the chopping control. The switching elements may be unexpectedly turned off by such noise immediately after the turn on if such chopping control is based on the target values (Vr1, Vr2) as explained. This problem may happen more frequently under the hard chopping mode if compared to the soft chopping mode because greater noise may be generated under the hard chopping mode. The chopping control may be affected by the noise for longer period of time till the regenerative current is weakened.